In recent years, hybrid vehicles (HV), plug-in hybrid vehicles (PHV), and electric vehicles (EV) are being spread. Secondary batteries as a key device are installed in these vehicles. As secondary batteries for the vehicle, the nickel hydride batteries and the lithium ion batteries are spread. In the future, it is expected that spread of the lithium ion batteries having high energy density are accelerated.
Since the operable voltage range and the prohibited voltage range in the lithium ion batteries are close, the stricter voltage management is necessary in the lithium ion batteries than other types of batteries. When an assembled battery in which a plurality of the lithium ion battery cells are connected in series is used, a voltage detection circuit is provided for detecting each of the battery cells. The voltage detected in each of the battery cells is used for controlling of charge or discharge, equalization in the cell voltages, or the like.
In order to achieve a higher voltage or a higher capacity, a plurality of assembled batteries are connected in series or parallel. In this case, cell balance among the whole of the plurality of assembled batteries, is required. A communication occurs, between one voltage detection circuit which manages one assembled battery and another voltage detection circuit which manages another assembled battery. Another communication occurs, between the voltage detection circuit and a control circuit. These communication loads are generally different, depending on the voltage detection circuits. Power consumptions in the voltage detection circuits are not equal.
In a case where a power supply of the voltage detection circuit is received from the assembled battery which is managed by such a voltage detection circuit, when power consumptions in the voltage detection circuits are not equal, the voltages or remaining capacities in the assembled batteries become not equal. Therefore, the following method is considered (for example, refer to Patent Literature 1). A discharge circuit for adjusting power consumption is connected to each of the assembled batteries, and thereby the voltages and remaining capacities of the assembled batteries are equalized.